bad error message on trove database initialisation failure

[Tom Fifield]

When there is a

DatastoreDefaultDatastoreNotFound

or

DatastoreDefaultVersionNotFound

exception raised on trove database creation attempt through horizon, only this is shown to the user:

"""
 Error: There was an error submitting the form. Please try again.
"""

This is not useful in terms of fixing the problem, and should provide more information.

Example debug log:
  File "/opt/stack/trove/trove/common/wsgi.py", line 251, in execute_action
    **action_args)
  File "/opt/stack/trove/trove/openstack/common/wsgi.py", line 395, in execute_action
    return self.dispatch(self.controller, action, request, **action_args)
  File "/opt/stack/trove/trove/openstack/common/wsgi.py", line 404, in dispatch
    return method(*args, **kwargs)
  File "/opt/stack/trove/trove/instance/service.py", line 186, in create
    datastore_models.get_datastore_version(**datastore_args))
  File "/opt/stack/trove/trove/datastore/models.py", line 179, in get_datastore_version
    datastore.name)
DatastoreDefaultVersionNotFound: Default version for datastore 'Legacy MySQL' not found.

Steps to reproduce.

1. Install devstack with all services enabled, using milestone-proposed branch
2. try to create a new database through horizon